1. Field of the Invention
The present invention relates to a push-button type tuner comprising a tuning slide member co-operating with a tuning core, a rotatable cam member engageable with said tuning slide member and lock means for positioning the cam member in the rotation direction thereof. More particularly, the present invention relates to a pushbutton type tuner having improved lock means.
2. Description of the Prior Art
Lock means of a cam member in the conventional pushbutton type tuner will now be described with reference to FIG. 10.
A plurality, for example, five, of V-grooves 2 are consecutively formed on one side edge of a tuning slide plate 1 capable of sliding in the direction indicated by arrow A co-operatively with a tuning core (not shown). On the front side of the V-grooves 2, a locking slide plate 3 is arranged so that it can move in the direction indicated by arrow B, which is rectangular to the sliding direction A of the tuning slide plate 1.
At a predetermined position of the locking slide plage 3, a cam bearing portion 4 having a V-groove shape expanded to the right in the drawings in arranged, and on the right side of the cam bearing portion 4, an action point 6 of a lever piece 5 is mounted with a certain space from the cam bearing portion 4. The top end portion of the lever piece 5 is connected to the locking slide plate 3 to form a fulcrum 7. The free end of the lever piece 5 acts as the force point, and in the vicinity of this free end, a locking pin 9 capable of moving in the direction of arrow B is inserted.
A cam shaft 11 connected to the lower face of a cam 10 is inserted between the cam bearing portion 4 of the locking slide plate 3 and the action point 6 of the lever piece 5. An engaging projection 12 is formed on the top end portion of the cam 10 so that it falls in abutting contact with the V-groove 2 of the tuning slide plate 1.
Before the presetting operation is performed, the locking slide plate 3 is pulled to the innermost position, and the locking pin 9 and lever pieces 5 are located at the positions indicated by dotted lines and the action point 6 of the lever piece 5 is separated from the cam shaft 11. Accordingly, the cam shaft 11 is not held between the cam bearing portion 4 and the action point 6 but the cam 10 is kept rotatable.
If the locking slide plate 3 is slid toward the tuning plate 1 in this state, the engaging projection 12 of the cam 10 is first caused to abut against the inclined portion of the V-groove 2 of the tuning sliding plate 1 preliminarily positioned by the manual operation. When the locking slide plate 3 is further advanced, the engaging projection 12 is guided by the inclined portion of the V-groove 2 while the cam 10 is being rolled, and the projection 12 is brought into abutting contact with the bottom 2a of the V-groove 2. If the locking slide plate 3 is further pushed in this state, the locking pin 9 is pushed on the side of the force point 8 of the lever piece 5, and the lever piece 5 is bent toward the cam bearing portion 4 with the fulcrum 7 being as the center as indicated by a solid line in the drawings and the cam shaft 11 is held between the cam bearing portion 4 and the action point 6. Thus, positioning of the cam 10 in the direction of rotation is accomplished and the presetting operation is completed.
When a preset station is selected, the locking slide plate 3 which has been cam-locked in the above-mentioned manner is pressed toward the tuning slide plate 1 kept in a freely slidable condition. By this pushing operation, the engaging projection 12 of the cam 10 is brought into contact with the inclined portion of the V-groove 2 of the tuning slide plate 1, and if the locking slide plate 3 is pushed, the engaging projection 12 arrives at the bottom 2a of the V-groove 2 while the tuning slide plate 1 is slid, whereby positioning of the tuning slide plate 1 is effected. Even if the locking slide plate 3 is further pushed in this state, the engaging projection 12 is not advanced any longer though the engaging projection 12 is strongly pressed to the bottom 2a of the V-groove 2. The tuning core is moved with the above-mentioned sliding movement of the tuning slide plate 1 and a desirable frequency is set.
In the above-mentioned conventional tuner, positioning of the cam 10 in the direction of rotation is effected by holding the cam shaft 11 between the cam bearing portion 4 and the action point 6 of the lever piece 5. The diameter of the cam shaft 11 is restricted to some extent by the width of the locking slide plate 3 or the lever piece 5 and it is not allowable to increase the diameter of the cam shaft 11 beyond a certain limit. Accordingly, in the conventional structure where positioning of the cam 10 is effected by holding the cam shaft 11 having a relatively small diameter between the cam bearing portion 4 and the action point 6, the engaging projection 12 formed on the top end portion of the cam 10 is apt to totter and shear in tuning is often caused. Accordingly, the conventional structure involves a problem concerning the reliability.
If the diameter of the cam shaft 11 is increased, the cam 10 is held relatively stably. However, since the width of the locking slide plate 3 or the lever piece 5 should be reduced for compensating the increased diameter of the cam shaft 11, the mechanical strength of the locking slide plate 3 or the lever piece 5, especially the force for holding the cam shaft 11, is reduced and undesirable results are often observed.